Simian immunodeficieny virus isolated from Macaca nemestrina (SIVmne), like HIV, evolves from a slowly replicating, minimally cytopathic, nonsyncytium-inducing (slow-low/NSI) virus to a population of rapidly replicating, highly cytopathic, syncytium-inducing (rapid-high/SI) viruses during the course of an infection. Because the changes in viral phenotype are associated with high viral loads, CD4+ T-cell decline, and progression to AIDS, they are believed to be indicative of increased virulence. The primary goal of this study is to determine whether the rapid-high/SI viruses are more pathogenic than slow-low/NSI viruses, and to identify the determinants responsible for the change in phenotype. Our previous studies suggested that the phenotypic switch in SIVmne was not the result of mutations in the envelope surface glycoprotein (Env SU). Therefore, to define the viral determinants responsible for the change in phenotype, we molecularly cloned a rapid-high/SI variant (designated SIVmne170) from the peripheral blood mononuclear cells (PBMCs) of a pigtailed macaque that was inoculated with a slow-low/NSI molecular clone, SIVMneCL8. Compared with the parent SIVMneCL8, SIVMne170 was syncytium-inducing, rapidly replicating, and highly cytopathic. The genetic determinants that conferred SIVMne170's phenotype mapped to the viral core proteins, Gag capsid and nucleocapsid, and the Env SU and transmembrane (TM) domains. We also cloned a second variant virus (designated SIVMne027) from lymph node tissue of a macaque. Interestingly, while SIVMne027 was also rapidly replicating and highly cytopathic compared with SIVMneCL8, it was nonsyncytium-inducing. Furthermore, SIVMne027 also had the capacity to replicate in nonstimulated macaque PBMCs. To test the pathogenicity of SIVMne170 and SIVMne027 in vivo, we infected juvenile pigtailed macaques by intravenous inoculation. Our preliminary data demonstrate that both variants may be more pathogenic than the slow-low/NSI parent virus, and that the SI property may not necessarily influence pathogenesis. Instead, the in vitro replicative and cytopathic properties may be more predictive of virulence in vivo. Together, these studies may provide insight into the significance that mutations in Gag and Env TM play in influencing SIV pathogenesis.